1. Field of the Invention
The present invention relates to a manipulating device aiming at the production of four-degree-of-freedom displacements of a rigid body.
2. Description of the Prior Art
Four-degree-of-freedom displacements were first studied by the German mathematician-mineralogist Arthur Moritz Schönflies (1853–1928), who found that these displacements have the algebraic structure of a group. For this reason, the set of such motions is known to geometers as the Schönflies subgroup of the group of rigid-body displacements. A familiar instance of Schönflies motions is found in the motions undergone by the tray of a waiter: the tray is free to move in all directions, but is prevented from tilting. The displacements comprise three independent translations and one rotation about an axis of fixed orientation with respect to a fixed base.
Schönflies motions are suitable for assembly tasks of the type occurring in the electronics industry. In these tasks, electronic chips having a flat geometry are picked up from a magazine, where they lie in a planar array. The chips are then placed on an electronic board in a planar array as well, the two planes involved being parallel but lying at different levels, the arrays thus having different orientations. These tasks require free translations in the three directions of space plus one free rotation about an axis of orientation fixed with respect to the plane of the magazine. The mechanism realizing these tasks must therefore exhibit a very large stiffness against rotations about two axes normal to the axis of rotation. These features motivate the name SCARA for this type of devices: Selective-Compliance Assembly Robot Arm.
Schönflies-motion generators, or SCARA systems, are not new. The Russian Patent SU921848 of Sep. 25, 1980 to Zharkov et al., discloses a redundant manipulating device that can be used for the generation of Schönflies motions, but through a serial layout of links and motors.
U.S. Pat. No. 4,610,598 issued on Sep. 9, 1986 to Hamada et al. discloses an industrial robot implementing Schönflies motions. The industrial robot has a kinematic chain of a serial manipulator, with the four axes of the Schönflies motions being actuated by motors traveling with the moving links. This mounting of the motors on the moving links imposes a high inertia load on the links, bringing about a low load-carrying capacity and a high flexibility of the structure. The high flexibility prevents high-speed operations because these operations induce structural resonance in flexible structures. As opposed to a serial layout, current industrial trends point at parallel arrays because they allow for a placement of the motors on a fixed base, thereby allowing for lighter, stiffer, and hence, faster structures.
A parallel array of serial chains producing individually Schönflies motions are found in U.S. Pat. No. 4,976,582 issued on Dec. 11, 1990, to Clavel, and in EP patent EP1084802 issued on Mar. 21, 2001, to Company et al., as well as in a number of similar devices appearing in the technical literature. The outcome of the patent to Company et al., like that to Clavel, is that the coordinated motion of various legs coupled to a common moving platform is lacking the rotation required in a Schönflies motion, because the rotations of the Schönflies motions of the individual legs are incompatible since their axes of rotation are nonparallel. In order to produce Schönflies motions of axis normal to all four motor axes, which are all parallel to a given plane, Company et al. resort to a planar linkage carrying the moving platform that hence undergoes Schönflies motions. Various embodiments of this invention have been reported in the Proceedings of the IEEE International Conference of Robotics and Automation in 2001, 2002, and 2003.
U.S. Pat. No. 6,095,011 issued on Aug. 1, 2000, and U.S. Pat. No. 6,301,988 issued on Oct. 16, 2001, both to Brogårdh, disclose devices intended to enhance Clavel's patent by having an axis of rotation normal to the coplanar axes of the Schönflies motions of the legs. However, none of these two inventions provides four degrees of freedom of the Schönflies type.
U.S. Pat. No. 6,336,374 issued on Jan. 8, 2002, to Brogårdh, discloses an improved version of his previous patents, providing for translations in three directions, as well as rotation around one of the axes. Here, the rotation is provided with a separate motor, installed on a first link of a manipulator with a long and intricate transmission between the motor and the rotating device. In a four-degree-of-freedom embodiment, two motors are installed on a base of the structure, and two are installed on a moving link. Thus, this embodiment represents a relatively complex, heavy and, consequently, slow device.
Several U.S. patents, namely, U.S. Pat. No. 4,437,635 issued on Mar. 20, 1984, to Pham; U.S. Pat. No. 5,746,093 issued on May 5, 1998, to Poglitsch; U.S. Pat. No. 5,397,323 issued on Mar. 14, 1995, to Taylor et al.; U.S. Pat. No. 5,419,674 issued on May 30, 1995 to Chang; U.S. Pat. No. 4,782,726 issued on Nov. 8, 1988 to Ryder et al.; and U.S. Pat. No. 5,931,832 issued on Aug. 3, 1999, to Jensen, disclose a variety of devices, all based on two serial parallelogram linkages, moving in the same plane, thus providing planar two-degree-of-freedom motion. Some of the foregoing inventions provide also additional motion of a base with the help of an additional motor. None of these inventions provides four-degree-of-freedom motions of the Schönflies type.
U.S. Pat. No. 5,536,135 of Jul. 16, 1996 to Robertson discloses a serial device with two orthogonal parallelogram-arrays, but only with two drives, thus allowing for two-degree-of-freedom translations with constant orientation of a tool.
Two four-degree-of-freedom parallel mechanisms, by Rolland, are described in Vol. 67, pp. 831–844 of ASME Journal Dynamic Systems and Control Division, of 1999. These two devices provide Schönflies motions. In one of these mechanisms, dubbed Kanuk, the author suggests to use four linear motors driving lower ends of links along rails. The second mechanism, dubbed Manta, uses three linear motors also driving the lower ends of links along the rails, and a fourth rotation motor installed on one of the links. These two devices thus need a very large space for installation and motion of the legs, providing at the same time a rather small workspace.
Finally, 50 parallel mechanisms capable of generating Schönflies motions were disclosed by Yang et al. in “Structure Synthesis of a 4-dof (3-translation and 1-rotation) Parallel Robot Mechanisms Based on the Units of Single-Opened-Chain”, Proceedings of the ASME 2001 Design Engineering Technical Conferences and Computers and Information in Engineering Conference, DETC2001/DAC-21151, Pittsburgh, Pa., U.S.A. Some of the mechanisms disclosed have legs with a single parallelogram, and the other mechanisms have a configuration without any parallelograms.